The present invention relates to a PVD coating device with an evacuable chamber,
which is equipped with at least one gas feed connection and
in which at least one target cathode, which is exposed to a sputtering process, at least one anode and at least one substrate holder which is intended to hold at least one substrate and is electrically connected to each substrate which is put in it, are arranged, and
with a control device having a first voltage output connected to deliver a first voltage in order to supply the target cathode with a negative electrical potential relative to the anode in order to form a plasma in which the substrate is arranged, and
having a second voltage output connected to deliver a second voltage in order to supply the anode with a positive electrical potential relative to the chamber wall,
as well as to a PVD coating method for producing a coated substrate. Such a PVD coating device is known from EP 0 434 797.
Generally, known PVD coating devices are used to provide all kinds of tools and components with coatings in order to give the surface a functional and possibly also decorative configuration. In the case of tools, these are predominantly coatings using metal-containing components, for example TiAlN.
One known technique for the production of coatings is the deposition of condensates from the gas phase. Numerous devices are used for this, each of which is optimized for particular methods. Devices in which the coating is made up of a high proportion of ionized atoms from a plasma are particularly advantageous for forming the coating.
In coating devices with which a high proportion of metal ions is produced in the plasma, a metallic material is regularly evaporated from the target cathode, and high proportions of the evaporated material are ionized. In devices of this type, the target cathode material is firstly melted before it enters the gas phase. The degree of ionization of the evaporated material is high.
However, in known devices of this type, in which the target cathode material passes through a molten phase, the disadvantage is found that, during the evaporation of alloys in the gas phase, droplets are formed and the gas phase is not homogeneous. Accordingly, the condensed coatings contain so-called droplets and/or the composition of the coating is rendered inhomogeneous.
Other known coating devices are configured in such a way that, in order to evaporate the target cathode material, cathodic sputtering takes place, with a magnetron old being used to increase the ionization efficiency. In this case, the material is converted directly from the solid state into the vapour state, without it being in the molten state in between.
However, cathodic sputtering devices have the disadvantage that the evaporated material is only weakly ionized. The plasma consists predominantly of evaporated neutral particles (degree of ionization about 5%) and other ionized gas particles which originate from working gases for ejecting target atoms and ions and for producing the plasma, or from reactive gases which bond with the target cathode materials. In the case of rough technical surfaces, in particular ket abraded or ground ones, the coatings deposited from these plasmas have disadvantages in terms of their properties of adhesion, hardness, structure and surface topography (smoothness and colour). In the case of such substrate surfaces, it has to date not been possible to produce a so-called dimpled surface coating which is distinguished by a dense, compact structure with smooth surface.
WO 91/00374 discloses a process and a device for coating substrates, in which both arc discharge volatilization and cathodic evaporation are used, the arc discharge volatilization being carried out before the cathodic evaporation. The ciruitry according to the WO 91/00374 includes a voltage source connected between a cathode and a chamber wall, a voltage source connected between the cathode and an anode and a voltage source for supplying a substrate with a bias voltage. An almost identical device is disclosed in the EP 0 558 061 A1.
The EP 0 677 595 A1 discloses a device for an arc discharge volatilization only. The device includes a single voltage supply, in which the cathode is connected continuously to the voltage supply, while optionally the anode or the substrate are connected to the remaining pole of the voltage supply.
On the basis of this prior art, the object of the invention is to further develop a PVD coating device of the generic type in such a way that the material of the target cathode evaporates from the solid phase without a molten phase, and, this being the case, a high proportion of the material condenses with good adhesion on the substrate, it also being possible for the substrate to be provided with a rough, in particular ground or jet abraded, technical surface.
This object is achieved in the case of a device of generic type in that the control device has a third voltage output connected to deliver a third voltage which supplies the substrate with an electrical potential that is more negative than the potential of the anode.
The effect achieved by this is that, inside the chamber and during operation of the PVD coating device, a further electrode, namely the substrate, is provided in the plasma with a defined potential, this electrode being arranged inside the plasma and ensuring an advantageous potential profile in the chamber. In particular, the second voltage, namely he anode voltage, makes it possible to set the current flowing at the substrate, while the third voltage, namely the substrate or bias voltage, can remain constant. The coating conditions on the substrate can thus be optimized particularly simply.
This electrical connection scheme also has a decisive effect on the electron density distribution in the chamber. The fact that the potential of the substrate is more negative than the potential of the anode achieves the effect that the trajectories of the electrons extend predominantly either, in order to sustain the plasma, from the target cathode to the anode or, to enhance the ionization of the metal atoms ejected from the target cathode, from the target cathode to the substrate. The enhanced ionization results from the transport path of the metal atoms from the target cathode to the substrate coinciding with the latter portion of the electron trajectories.
By virtue of the potential profile set up according to the invention in the chamber, an ionization efficiency of up to 50% can be achieved for the ejected target metal atoms. As a consequence of this, using this coating device in a sputtering method, a degree of ionization will be achieved which is suitable for depositing layers with a dimpled surface on the substrate, even if the substrate surfaces are rough, for example because of a grinding or jet abrasion treatment. A power density of up to 45 W/cm2 can be achieved at the target cathode, and the discharge current is increased considerably in comparison with the prior art.
The potential profile in the chamber also has the further result that the potential of the plasma ignited between the target cathode and the anode in the region of the substrate is fundamentally more positive than the substrate potential. The effect of this is that, for example, ions originating from the plasma in this region of the chamber are transported with a high probability in the direction of the substrate and can be deposited there.
Electrons are furthermore prevented from reaching the chamber wall and therefore being ineffective for ionizing these metal atoms. For this purpose, the substrate preferably has the same potential as the target cathode, or is biased positively relative to the target cathode, so that electrons are drawn in the direction of the substrate in order to ionize the target material. It is, however, also possible for the potential of the target cathode to be more positive than the potential of the substrate.
It is also possible to provide a plurality of anodes and target cathodes which are arranged symmetrically in such a way that the substrate lies in the plasma during the coating process.
The substrates are, for example, cutting tools which are to be provided with a coating of hard substance.
Preferably, the second and third voltages are set in such a way that the positive electrical potential of the anode with respect to the chamber wall is less than the positive electrical potential of the anode with respect to the substrate, and in that the substrate is arranged in the vicinity of the target cathode.
In this embodiment of the invention, the optimization of the electron density distribution in the chamber before the target cathode is achieved particularly efficiently. By virtue of the negative bias voltage of the substrate with respect to the anode and the proximity of the substrate to the target cathode, the proportion of electron trajectories leading to the substrate is further increased, and the ionization of the metal atoms from the target cathode is therefore improved.
Preferably, the first, second and third voltages are set in such a way as to set a floating potential which, when the plasma is ignited between the target cathode and the anode in the region of the substrate, is about 40 volts to about 400 volts, preferably 130 volts, higher than the potential of the substrate.
This ensures that ions originating from the plasma in the region of the substrate pass very predominantly in the direction of the substrate and, on the substrate, can be deposited in order to form a coating. The losses of ions resulting from their recombination on the chamber wall are therefore minimized.
In a preferred embodiment, the first voltage is set in such a way that the anode is at a potential which is up to about 800 volts higher than the potential of the target cathode. The second voltage may be set in such a way that the anode is at a potential which is between about 50 volts and up to 250 volts more positive than the potential of the chamber wall, while the third voltage is preferably set in such a way that the anode is at a potential which is up to 800 volts, in particular 100 volts to 200 volts, more positive than the potential of the substrate.
These value ranges for the potential differences between the anode, target cathode, chamber wall and substrate have been found experimentally to be particularly favourable. When setting the potentials, in particular the potential between the anode and the cathode, the respective properties of the target materials should be taken into account, in particular including their magnetic properties.
The ratio of the distance between the target cathode and the substrate to the distance between the target cathode and the anode is preferably about 1:5.
This distance ratio leads to a geometry for the electrodes, target cathode, substrate, chamber wall and anode, which is distinguished by a particularly favourable profile of the equipotential lines which determine the trajectories of the charge carriers and, in particular, the electron density distribution.
The substrate holder may, for example, be arranged in such a way that the substrate is arranged approximately at a distance of 40 mm from the target cathode, and the distance between the target: cathode and the anode is in the region of 250 mm.
A particularly compact structure of the PVD coating system is achieved in this way. In particular, the short distance from the target cathode to the substrate leads to very high coating rates for coating the substrate. This results in economically favourable production of the coating by virtue of high-speed production.
Preferably, the control device has a voltage source assigned to each of the three voltages, the first voltage source being connected between the anode and the target cathode, the second voltage source being connected between the anode and the chamber wall and the third voltage source being connected between the anode and the substrate. Using the second voltage source, the current at the substrate is in this case set by acting on the plasma, while by retaing the setting of the third voltage source, the potential difference between the substrate and the anode will be maintained even if the setting of the second voltage source and therefore the substrate current is altered.
In this case it is also ensured that any possible fluctuations in the anode potential do not have repercussions on the voltage differences between the anode and the target cathode, and between the anode and the substrate, so that more stable operation of the PVD coating device is achieved.
It is, however, also possible for all three voltage sources to have one of their poles on the chamber wall.
As an alternative to the embodiment with three voltage sources, it is possible for the control device to have only one voltage source, which delivers the first voltage between the anode and the target cathode, and to have, for the second and third voltages, a respective variable resistor which is connected in parallel with the voltage source. The variable resistors are set in such a way that the required potentials for the anode and the substrate are present on their respective taps.
All the same, irrespective of the manner of electrical connection, what is essential to the invention is that the above described potential profile in the chamber is maintained.
The abovementioned object is likewise achieved by a PVD coating method for producing a coated substrate, comprising the following steps:
arranging at least one target cathode, which is exposed to a sputtering process, at least one anode and the substrate, which is arranged on a substrate holder which is electrically connected to the substrate, in an evacuable chamber;
filling the chamber with a working gas and/or a reactive gas through at least one gas feed connection;
applying a first voltage in order to supply the target cathode with a negative electrical potential relative to the anode in order to form a plasma in which the substrate is arranged;
applying a second voltage in order to supply the anode with a positive electrical potential with respect to the chamber wall;
applying a third voltage which supplies the substrate with an electrical potential more negative than or equal to that of the anode; and
removing the substrate from the chamber after a predetermined coating time.
The essential step in this production method for a coated substrate consists in the procedural step of applying the third voltage, as described above. This procedural step considerably increases the electron density before the target cathode, and is therefore decisive for the improved quality and faster production of the coating in comparison with the prior art. The method can advantageously be developed further by employing the above-described features of the coating device.